PSI - Issue 82

S. Belodedenko et al. / Procedia Structural Integrity 82 (2026) 260–266 S. Belodedenko et al./ Structural Integrity Procedia 00 (2026) 000–000

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Fig.2. Comparative LAD of the 09Cr16Ni4Nb steel when K σ =1 (1), 13Cr15Ni4 Мо 3 steel when K σ =1 (2), when K σ =4 (2, К =4), М 18 bolts made of this steel with knurled thread ( М 18) in the cycle asymmetries positive.

Fig.3. Experimental models of the relationship between cyclic fracture toughness and cycle asymmetry of steels 09Cr16Ni4Nb steel (left), 13Cr15Ni4 Мо 3 steel (right). 3. Results and discussion The authors explain the investigated phenomena from the standpoint of the merging of fatigue and fracture mechanics concept. The main connections between the two theories were formulated by J.C. Newman (1998). The loss of sensitivity to cycle asymmetry for high-strength steels can be explained by a similar phenomenon for the parameters of the Paris diagrams. With an increase in cycle asymmetry after a certain R , the values of the threshold Δ K th and critical Δ K fc cease to decrease Fig.4. Moreover, the ranges of change of Δ K fc are less than the range of Δ K th . This feature is associated with the phenomenon of crack closure. The relationship between the values of Δ K th and the amplitude of the endurance limit σ а is carried out through the σ a ( Δ K th , K σ ) function, the form of which depends on the loading method, the level of stress concentration or the sharpness of the notch Fig.4. Therefore, according to Petr Lukáš and Ludvík Kunz (1981), this relationship can have a different form. As can be seen from the LAD formation diagram Fig.4, the form of the functions Δ K fc (R), Δ K th (R), σ a ( σ m ) are similar to each other. The concave nature of the LAD is explained by the existence of some minimum possible endurance limit σ ath , which corresponds to the value of Δ K th . Therefore, the LAD line cannot reach the σ m = σ y or σ mr = 1 points. In fact, the limit of existence of the LAD from the straight line between the points σ а r =1 to σ mr =1 (limit 1, Fig.2) goes into the limit σ а = σ ath (limit 2, Fig.4). Below this line, the mechanism of material failure changes. The material can enter the zone of so-called static fatigue with failure from plasticity exhaustion. There emerges a transition from creep-fatigue